A
variety of pilot switches is available for use in control circuits. One
of the most popular is called a limit switch (see Figure right). These
switches are mounted physically in the machine so that motion of the
machine or the parts around the switch will cause the switch to activate.
Figure 1a shows a yoke limit switch. The yoke has a roller on each section
of its activator arm. This arrangement is used where machine motion is
in two directions. When the machine moves in one direction, it strikes
the right-hand roller and causes it to switch to the right. This action
causes the right roller to be pressed downward, and the left roller snaps
to the up position where it's in position to detect machine motion when
it moves back to the left. The yoke arm on switch will continue to detect
motion to the right, and then to the left. This type of switch is useful
for surface grinders where the part being finished is moved back and forth under the grinding wheel. Each time the part moves to the end of
its stroke, the switch is activated to the other direction.

Figure 1b shows a limit switch with a single roller arm. This type of
switch detects motion in only one direction. When the machine moves past
the roller arm, it will move the arm to the right so that the switch
is activated. Figure 1c shows a top roller limit switch. This type of
switch has a small roller mounted on a plunger arm. When the machine
moves past the roller, it causes the plunger to depress to activate the
switch. When the machine moves past the switch, a spring causes the plunger
to move back upward into place to be reactivated. This type of switch
can detect motion in either direction. A variation of this type of switch
uses a blunt end on the plunger, which means the switch can only detect
motion that is directed down on the plunger. This type of switch is generally
mounted at the very end of machine travel, and when the machine reaches
the end of its stroke, it depresses the plunger.

Figure 1d shows a wobble lever actuated limit switch that is sometimes
called a cat whisker limit switch. This type of limit switch can detect
motion in any direction. That is, any movement of the cat whisker can
cause the switch to activate. This type of switch is also much more sensitive
than other types of limit switches. Figure 1e shows a side roller limit
switch. This type of limit switch is mounted in such a way as to detect
end of travel. When the machine reaches the end of its travel, it will
depress the plunger and cause the switch to activate.